The first point I would like to make, that half of you don't seem to get: I'm not asking if I should be upset because she isn't a virgin. I'm not. I am asking if I should be concerned because she lied about it.
I wouldn't say that discussing her past on a forum is that huge of a deal. I barely know any of you, and none of you know me personally. And even if you did, you would have no idea I am who I am. Sometimes the best people you can ask about a problem are people that you barely know at all that have no connection with you personal life- which is, in this situation, you guys.
Besides, this could all be a hypothetical situation for all you know. I COULD be the girl wondering if my boyfriend is going to dump me, so I asked from his perspective.
Also, when I asked my RL friends about it, it was only three of them, and I've known these guys for 4+ years and know they'd never betray my trust. Plus, they haven't spent a lot of time around my girlfriend and they don't know her- well actually, one of them does know her. They have asked me the same type of questions, and it's moreso of a friendship wing-man help-me-out understanding, rather than a talk-behind-someone's-back understanding.
And the third thing is, I asked because in case anything ever happened (which happens quite a lot in high school, in fact), I wanted to know if she'd been with anyone. Because like one of you guys said earlier, STD's and junk fly around, and though it isn't likely my 16 year old girlfriend is going to have AIDS, people still buy lottery tickets in hopes that they will win, and that's even more unlikely than my girlfriend having an STD. So, if you've ever bought a lottery ticket, you can't go against my judgement.